1. Field of the Invention
The present invention relates to a phase-change recording method suitable for recording on a phase-change recording medium, which can be in a multiplayer structure and is large in transmission factor (transmittance) like an optical disc enabling high-density recording, by utilizing a rewritable recording phase-change medium.
2. Description of Related Art
Nowadays, as information amount growingly expands, demands for recording mediums which can record/retrieve a large capacity of data at high density and high speed have been on the rise. Therefore optical discs have been expected to meet these growing demands. Optical discs are divided into two types: one type enabling recording for only one time, and the other type enabling recoding/erasing for many times. The rewritable optical discs are exemplified by a magneto-optical recording medium utilizing a magneto-optical effect, and a phase-change medium utilizing change in reflectance with reversible change in crystalline state.
This phase-change recording medium can make recording/erasing only by modifying laser light power (output) without outer magnetic field and enables downsizing a recording/retrieving apparatus. Further, if the phase-change recording medium is used, it is possible to realize recording/retrieving information in high density by a short-wavelength light source without changing the material of a recording layer in particular from a medium that is recordable/erasable by light power about 800 nm in wavelength, which is most popular in the art.
Thin films of chalcogen alloy are used for the recording layer material of many of commercially available phase-change mediums. This chalcogen alloy is exemplified by GeSbTe alloy, InSbTe alloy, GeSnTe alloy, and AgInSbTe alloy. In the currently practical recording method for a rewritable phase-change recording medium, the recording layer takes a crystalline state as unrecorded/erased state, and an amorphous bit is formed for recording. An amorphous bit is formed by heating the recording layer up to a temperature higher than a melting point and then rapidly cooling down the recording layer.
In order to prevent possible vapor and deformation, which might occur due to the heating of the recording layer, the recording layer is ordinarily sandwiched a set of upper and lower dielectric protective layers which are resistant to heat and chemically stable. Further, in general, a metallic reflective layer is placed on the sandwich structure to provide a quadri-layer structure so that heat dispersion is facilitated and amorphous marks are formed stably.
This metallic reflective layer serves to escape heat generated when the recording layer is irradiated by a recording laser light beam (hereinafter also called xe2x80x9clight beamxe2x80x9d). Namely, if an amorphous substance is used in a phase-change medium, the recording layer is locally melted by the light beam, and then the resulting recording layer is rapidly cooled to form an amorphous mark. In the presence of inadequate radiation, this amorphous mark cannot be formed neatly as intended; consequently the metallic reflective layer is required.
In order to form the amorphous mark stably, the divided pulse method has been customary to divide a mark-forming laser pulse. On many occasions, assuming that a reference clock period is T, a pulse sequence of the period T is irradiated according to the mark length. At that time, to make the temperature distribution in the mark uniformly, the time length of the leading pulse (the first pulse) is set to larger than that of the second and subsequent pulses.
The divided pulse method is a method of forming an amorphous mark of a time length nT by alternately irradiating the phase-change recording medium with a write power Pw having a relative high power value and a bias power Pb having a relatively low power value. Here n is a natural number equal to or larger than 4).
Specifically, of the output pattern (pulse pattern) of the light beam, a writing pulse to be output at high power is divided into a plurality of pulses, and an off-pulse to be output at low power is divided into a plurality of pulses; these high and low power irradiations are alternately repeated. In the conventional divided pulse method, for every pulse, the time length of the divided writing pulse to be output at high power and the time length of the divided off-pulse are nearly 0.5T.
The pulse patter when the amorphous mark is formed is disclosed in the following publications 1, 2 and 3. Publication 1 is xe2x80x9cThe Feasibility of High Data Rate 4.7 GB Media with Agxe2x80x94In-Sub-Te Phase Change Materialxe2x80x9d, (Collection of Theses presented in 10th Symposium of Phase Change Media Society 1998) disclosing a technology relating to the pulse pattern.
Publication 2 is xe2x80x9cRewritable Dual-Layer Phase-change Optical Diskxe2x80x9d, Jpn. J. Appl. Phys. Vo. 38(1999) pp. 1679-1686) disclosing a technology relating to an optical disk in the form of a rewritable phase-change medium having a multiplayer structure. Specifically Publication 2 describes a rewritable pulse pattern.
Publication 3 is Japanese Laid-Open Publication No. Hei 3-185628 (U.S. Pat. No. 5,109,373) disclosing a technology relating to a method and apparatus for recording signals on an optical information recording medium, such as an optical disk, for recording/retrieving optical information at high speed and in high density using a laser light beam. Specifically, Publication 3 describes a value satisfying a repeating period xcfx84 in an intermediate pulse sequence.
In the meantime, for erasing (crystallizing), the recording layer is heated up to a temperature higher than a crystallization point of the recording layer and lower than a melting point of the recording layer. In this case, the dielectric protective layer serves a heat storage layer to keep the recording layer at a high temperature enough for crystallization.
Further, in a 1-beam overwritable phase-change medium, the above-mentioned erasure/rewrite processes are carried out only by modifying a single focused light beam. This technology is disclosed in Publication 4 (Jpn. J. Appl. Phys. 26 (1987), suppl. 25-4, pp. 61-66). Furthermore, by using 1-beam overwritable phase-change medium, the layer structure of a recording medium and the circuit structure of a recording drive apparatus would be simple. Therefore a system using a 1-beam overwritable phase-change medium is watched with a keen interest for inexpensiveness, high density and large capacity.
Recently, attempts have been made to increase the number of layers of a recording medium to a much higher density. An attempt to increase the recording density is to manufacture two or more recording medium parts spaced from each other by a distance larger than the focus depth of an optical system being used. In this attempt, the recording medium parts except the farthest recording medium part, as viewed from the substrate where the laser light comes in, respectively require a high transmission factor of 30% or more to permit the laser light to pass.
Consequently, in order to permit the laser light, it is essential that basically no metallic reflective layer is used, or a metallic reflector has such a small thickness as to permit adequate light to pass.
Yet in the recording medium devoid of a metallic reflective layer or having a thin metallic reflective layer, since only inadequate heat radiation effect can be achieved, re-crystallization would tend to occur when an amorphous mark is formed, so that an amorphous mark neat as intended is difficult to form.
There could be another attempt to prevent re-crystallization by modifying the composition of the recording layer at least to make the crystallization speed slow. But because of the slow crystallization speed, an amorphous mark to be irradiated by the erasure power (hereinafter called xe2x80x9cerasure power irradiation sectionxe2x80x9d) after having been formed can be crystallized only inadequately so that the amorphous mark cannot be erased.
Namely, when making recording on a phase-change medium that is devoid of a metallic reflective layer or has a thin metallic layer, it is difficult to prevent re-crystallization of the mark during recording while keeping adequate erasure ratio at the erasure power irradiation section, thus narrowing the range of effective crystallization speeds for a rewritable recording medium.
With the foregoing problems in view, it is an object of the present invention to provide a recording method for a rewritable phase-change recording medium which method facilitates forming/erasing amorphous marks even if a reflective layer of the recording medium is only a limited thickness or void, without any risk of restricting the range of effective crystallization speeds.
In order to attain the above object, according to a generic feature of the present invention, there is provided a recording method for a phase-change recording medium having a phase-change recording layer in which amorphous marks each having a time length nT (T is the data reference clock period, and n is a natural number equal to or larger than 4) are formed by alternately irradiating the recording medium at least with a high-power energy beam having a relatively high power value and a low-power energy beam having a relatively low power value, said method comprising the steps of: (a-1) irradiating the low-power energy beam to the recording medium for a first set time yT (y is a natural number larger than or equal to 0) as a preceding low-power pulse irradiation step immediately before a prospective leading pulse that is a time section where the high-power beam is irradiated for the first time, and (a-2) irradiating the low-power energy beam to the recording medium for a second set time xT (x is a natural number larger than 0) as a succeeding low-power pulse irradiation step immediately after said prospective leading pulse, x and y having a relation expressed by the following formula
0.95xe2x89xa6x+0.7*yxe2x89xa62.5 
where * is an arithmetic symbol representing a multiplication; and (b) irradiating the high-power energy beam to the recording medium in such a manner that a period of irradiation for pulses subsequent to the leading pulse is in a range of from 0.5T to 1.5T.
According to this recording method of the present invention, it is possible to prevent re-crystallization of the marks during recording, which would have been encountered with a small-thickness phase-change recording medium, even if the phase-change recording medium is devoid of any metallic reflective layer. And since the conventional pulses except for the leading end portion of a divided pulse signal can be utilized without any special reconstruction, it is possible to facilitate circuit designing.
Further, it is possible to prevent re-crystallization of the marks during recording, with keeping an adequate erasure ratio at the erasure power irradiation section, even if information in terms of different mark lengths is recorded on a phase-change recording medium devoid of a metallic reflective layer or having a thin metallic reflective layer, thus eliminating the conventional problem that restricts the range of effective crystallization speeds.